Due Date Calculator
Estimate pregnancy due date
Due Date Calculator tool
Due Date Calculator: key facts
- What it does
- Estimate pregnancy due date
- Category
- Health & Fitness Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
What the Due Date Calculator does
For expectant parents, the estimated due date is the anchor everything else is planned around. This calculator applies Naegele's rule — the standard method clinicians use — adding 280 days to the first day of your last menstrual period to estimate when your baby is due.
It gives a quick, recognized estimate for early planning, appointments, and counting down the weeks. While an ultrasound and your healthcare provider give the most accurate dating, the last-period method this tool uses is the widely accepted starting point and matches how due dates are first calculated. Keep in mind that only about one in twenty babies arrives on the estimated date itself — the figure marks the middle of a window, not a deadline.
Using the Due Date Calculator, step by step
- Select the first day of your last menstrual period.
- Read your estimated due date, calculated as 280 days later.
- Use it as a planning anchor, and confirm timing with your healthcare provider.
Naegele's rule: 280 days from the last period
The estimated due date is calculated as 280 days — 40 weeks — from the first day of your last menstrual period. This is Naegele's rule, formalised in the early nineteenth century and still the standard starting point in obstetric practice worldwide.
The 40 weeks are counted from the last period rather than from conception, which is why gestational age always runs about two weeks ahead of the embryo's actual age. Conception typically occurs around day 14 of a 28-day cycle, so the fetus at 12 weeks gestational age has been developing for roughly 10.
Two assumptions are built in: that your cycle is 28 days and that ovulation happens on day 14. Both are averages rather than rules, which is the main reason a dating scan sometimes moves the date.
- A last period beginning 1 March gives an estimated due date of 6 December, 280 days later.
- Counting back, conception would fall around 15 March and the first trimester would end in late May.
- On a consistently 35-day cycle, ovulation is later and the true date is likely about a week beyond what this rule reports.
What makes this one worth using
- It uses Naegele's rule (last period plus 280 days), the standard method clinicians use to estimate due dates.
- It needs just one date and returns the estimate immediately, with no forms or sign-up.
- The result is a clear calendar date you can plan around.
- Your information is processed locally and never leaves your browser.
Only about one birth in twenty lands on the date
A due date is the midpoint of a distribution, not an appointment. Roughly one in twenty births occurs on the estimated date itself, while term is defined as the broad window from 37 to 42 weeks. Spontaneous labour most commonly begins in the two weeks either side, and first pregnancies tend to run slightly longer.
The rule is least accurate where its assumptions fail: irregular or long cycles, an uncertain date for the last period, conception soon after stopping hormonal contraception, or while breastfeeding. In those cases the calculated date can be out by a week or more. Twins and higher multiples also typically arrive earlier than a singleton calculation suggests.
An ultrasound in the first trimester dates a pregnancy more accurately than any calendar arithmetic, measuring the embryo directly, and where the two disagree clinicians generally revise to the scan. For IVF pregnancies the date is calculated from the known transfer date instead, which is more precise than either.
Treat this as an orientation for planning while your antenatal care establishes a clinical date. Any bleeding, pain, reduced fetal movement, or concern about the pregnancy needs a midwife or doctor promptly, and nothing here substitutes for antenatal care.
Frequently Asked Questions
How is a due date calculated?
The common method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. This calculator applies that rule directly. It assumes a regular 28-day cycle, which is why a provider may adjust the date.
How accurate is the estimated due date?
It is an estimate — only about 1 in 20 babies arrive exactly on their due date, and most are born within a couple of weeks either side. An early ultrasound often refines the date, especially if your cycle is irregular.
What if I don't know my last period date or have irregular cycles?
The last-period method is less reliable with irregular cycles or an uncertain date. In those cases, dating is usually based on ultrasound measurements, so check with your healthcare provider for a more accurate estimate.
Can the due date change later in pregnancy?
Yes. Providers often refine the date after an early ultrasound, which measures the baby directly and can be more accurate than the last-period estimate, especially with irregular cycles. The figure here is a starting point, not a fixed appointment.
